In the field of telecommunications, the technologies dubbed Universal Mobile Telecommunications System (UMTS), CDMA, CDMA2000 and CDMA 2000 1xEV-DO are based on a transmission coding entitled “Code Division Multiple Access” (denoted CDMA). This CDMA code is based on a spread spectrum technique (“Direct Sequence Spread Spectrum”). It allows several digital links to use the same carrier frequency simultaneously.
The spreading of the spectra by the CDMA code is carried out by means of a code allocated to each communication. The receiver uses this same code to demodulate the signal that it receives and extract the useful information. Each user uses a different code.
The receiving base station (in general denoted “node B” for UMTS technology) receives the sum of the spread signals of various users. The total signal received is then correlated with the spreading code of a desired user. The effect of this is to restore the signal of the desired user and to attenuate the signals of the other users. Given the spreading of the spectra at the base station considering only one user, all the other users are perceived as noise. For this reason, the network adjusts the output power of each user so as to receive them all at the same level for one and the same user service, that is to say all at the same signal/noise ratio.
If there is a significant difference in a signal/noise ratio between one user and the other users of one and the same cell of a base station, the base station will not be capable of decoding the other users. Indeed, the other users will be hidden by the user having the signal with the highest transmission power.